Rheumatoid arthritis (RA) is a common and extremely debilitating inflammatory disease that results in erosion and degradation of joint tissue and ultimately loss of function. Activation of the hemostatic system and the subsequent conversion of prothrombin to the active serine protease thrombin is a prominent feature of RA. Robust fibrin deposition within affected joints is one pathological consequence of exuberant thrombin activity, and fibrin(ogen) has recently been shown to drive inflammatory events associated with arthritis pathogenesis. Importantly, fibrinogen appears to be just one of many downstream effectors by which thrombin regulates events central to arthritis pathogenesis. The aims of this proposal will focus on the following specific hypotheses: (1) thrombin promotes the local inflammation, synovial hyperplasia, pannus formation, and cartilage/bone destruction associated with arthritic disease, (2) thrombin-induced fibrin polymerization and activation of the fXIII transglutaminase which cross-links fibrin are two mechanisms by which thrombin exacerbates inflammatory joint disease, and (3) re-engineering thrombin substrate specificity to promote anticoagulant/anti-inflammatory activity over procoagulant activity will convert the enzyme from a pro-arthritic to an anti-arthritic factor. These hypotheses will be tested using recently established gene-targeted mouse lines and the well-established experimental setting of collagen-induced arthritis. The specific effect(s) of a genetically-imposed reduction in circulating prothrombin in conjunction with fibrinogen-deficient mice will define the role of the thrombin-fibrinogen axis on arthritis pathogenesis (Aim 1). By employing mice that either express a form of fibrinogen insensitive to thrombin protease activity or that carry a genetic ablation of the transglutaminase fXIIIA catalytic subunit gene, the mechanistic role of thrombin-mediated fibrin polymerization and cross-linking on arthritis progression will be specifically evaluated (Aim 2). Finally, mice carrying a site- directed mutation in the prothrombin gene that results in a shift in substrate specificity away from procoagulant to anticoagulant/anti-inflammatory targets will be used to investigate the concept that thrombin can be redirected to serve as an anti-arthritic enzyme (Aim 3). The proposed studies will fill significant gaps in our current understanding of arthritis pathogenesis by defining mechanisms of cross-talk between the hemostatic and inflammatory systems that drive arthritis pathogenesis and may highlight novel therapeutic opportunities for the treatment of arthritis. PUBLIC HEALTH RELEVANCE: Activation of the hemostatic system, including generation of the central hemostatic serine protease thrombin, is a prominent feature of both human rheumatoid arthritis and experimental inflammatory arthritis. The long-term goal of these studies is to determine the role of thrombin in the pathogenesis of inflammatory joint disease. The proposed work will fill significant gaps in our understanding of the interplay between the thrombin- fibrinogen axis and arthritic disease, and may provide the proof-of-principle for the use of novel customized thrombin variants with selected substrate specificity to treat arthritis.